The gunsmiths rank should increase the light level of the guns you purchase on arms day. If you are rank 6 then the guns he drops are 286 instead of 280. At rank 40 the guns are 320. The current light leveling system is a mess. You get stuck at a light level for weeks of you don't get a drop. This would provide a direct predictable route to leveling guns and provide an alternative for solo players or people who have trouble getting raiding groups together to level.